As LSI circuits are increasing in density, circuit linewidths of semiconductor devices are becoming finer year by year. To form a desired circuit pattern on a semiconductor device, a method is used which includes reducing the size of a high-accuracy original pattern (mask or also called a reticle that is used, in particular, in a stepper or a scanner) formed on quartz and transferring the pattern to a wafer with a reduced projection exposure apparatus. The high-accuracy original pattern is written through an electron beam writing apparatus by means of so-called electron beam lithography technology.
In an electron beam wiring apparatus, thermoelectrons emitted from an electron source included in an electron gun of the apparatus are accelerated into an electron beam by an acceleration voltage, and the electron beam is applied to a sample. A known technique for increasing the brightness of an electron gun includes covering the surface of a material for forming the electron gun with a material having a higher work function than the material for the electron gun to reduce the area of electron emission of the electron gun. For example, the surface of lanthanum hexaboride (LaB6) constituting an electron source is covered with carbon (C), and electrons are emitted from a bare tip of the surface of lanthanum hexaboride.
Under high voltage application, abnormal discharge of an electron gun may be caused by discharge factors including a protrusion, such as a burr or a defect in the surface of an anode or a Wehnelt electrode, and contaminants, such as dust. Such abnormal discharge often occurs after mounting of a new electron gun, after replacement with a new electron gun, and after maintenance of an electron gun. To prevent abnormal discharge, an electron gun is typically subjected to conditioning (electrode electrical discharge machining) after mounting, replacement, or maintenance.
During conditioning treatment for an electron gun, discharge may occur between a Wehnelt electrode and a cathode, and contaminants including carbon may be deposited on the surface of the tip of an electron source. Under the electron gun, a current limiting aperture member made of, for example, tantalum or tungsten, is disposed. When the electron gun emits an electron beam, the current limiting aperture member may generate gas, and the generated gas may cause contaminants to be deposited on the surface of the tip of the electron source.
The contamination of the surface of the tip of the electron source with the contaminants obstructs the emission of an electron beam, degrading electron beam emission characteristics. Although heat treatment for removing such contaminants has been performed in the related art, the contaminant removal by heat treatment requires long time (over ten hours to several tens of hours), during which the operation of a writing apparatus needs to be stopped. This results in a reduction in availability.